The conventional technique for pumping oil from underground reservoirs utilizes a reciprocating downhole pump. At the surface, a motor drives a walking beam which is connected to a polished rod that is in turn connected to a string of sucker rods which extend down the borehole to support the downhole pump. As the motor runs, the walking beam raises and lowers the polished rod and string of sucker rods which causes the pump to lift the fluid from the reservoir up to the surface.
A number of different occurrences can take place the bottom hole location of the pump. The production rate of the well may not be sufficient to fill the pump, gas interference can cause poor pump performance, the pump can become jammed with sand or the pump can become defective. There also could be other downhole problems. Often the conditions at the downhole pump can be determined by measuring the load on the sucker rod at the surface is therefore important in well production to determine the load on the sucker rod to evaluate when the well should be pumped or to notify the operator when there is a problem that requires action. Also, the surface pumping equipment may be overloaded or even severely underloaded.
Several different types of load transducers have been developed. One such load transducer is shown in U.S. Pat. No. 4,932,253 to McCoy. This patent shows a doughnut type load cell. U.S. Pat. No. 3,355,928 to Neeley et al. describes a transducer which is placed into a spacer wherein the transducer has means for extension so that the load can be applied to the transducer without the need for separating the polished rod clamp from the carrier bar. Both McCoy and Neeley describe transducers in which strain gauges are attached to a transducer load member wherein the polished rod load is placed entirely upon the transducer.
Other types of load transducers are disclosed in U.S. Pat. No. 3,965,736 to Welton, U.S. Pat. No. 4,090,405 to McKee, and U.S. Pat. No. 4,644,785 to Doyle. Welton discloses a split cylinder which is used to measure the elongation of the polished rod in order to obtain loadings. McKee discloses a body having at least three symmetrical sections pivotally connected together with a clamping means for clamping the body about a polished rod. The McKee transducer senses the elongation of the rod in order to determine loading. Doyle describes an apparatus for measurement of elongation of the polished rod wherein the apparatus minimizes the transducer output due to bending of the polished rod. Thus, Welton, McKee and Doyle all describe devices which measure the elongation of the polished rod.
A need exists for a polished rod transducer which is lightweight, convenient to use, reasonably rugged, reasonably accurate and can be safely and easily attached to the polished rod in order to analyze the performance of the pumping system. The described polished rod transducer can be attached to commonly used 1 1/8, 1 1/4 and 1 1/2 inch polished rods conveniently and easily. The transducer can be easily attached to the polished rod anywhere below the carrier bar that remains clear of the stuffing box.